This invention relates to the production of polymers and copolymers and, more particularly, to the vapor state polymerization of a polymerizable monomer or mixture of monomers to produce normally solid polymeric substances. Specifically, the invention is directed to a method and apparatus for controlling the removal of polymeric product following the polymerization of the monomer or mixture from the vapor state by an essentially isobaric process using a high yield catalyst and, optionally, cocatalyst in a horizontal, quench-cooled, stirred-bed reactor preferably having essentially total reactor off-gas recycle.
One of the problems with solution or slurry polymerization of monomers is the capital cost required in the production of polymeric product. Monomer polymerization using a vapor state process can be considerably more economical if certain problems inherent in vapor state polymerization can be solved. These include problems of carrying out the polymerization in a thermally controlled fashion so as to avoid hot spots, maintaining a proper product particle size distribution, and, in the case where catalysts are used which have high yields but are extremely sensitive to poisoning, decreasing to a minimum the amount of make-up material seen by the catalyst per amount of product formed. Another problem related to certain catalyst combinations is the narrow molecular weight distribution of the products formed with these catalysts.
U.S. Pat. Nos. 3,965,083; 3,971,768; 4,101,289; and 4,129,701, assigned to the assignee of this application, disclose a horizontal reactor for the essentially isobaric, vapor phase polymerization of polymerizable monomers using essentially total reactor off-gas recycle and a quench-cooled, stirred-bed mode of operation. By the term vapor phase process or reactor is meant a process or reactor, the monomer or monomers of which are vapors or gases under the conditions prevailing in the reactor.
The reactor can have one or more polymerization sections, and, preferably, there are at least two sections separated from each other by weirs or other suitably shaped barriers to prevent gross back-mixing between sections. Each section can be individually controlled in terms of polymerization temperature and polymer production rate so that a polymeric product having a controlled spread of molecular weight and particle size can more easily be produced.
The reactor introduces catalyst components and quench liquid into the polymerization sections directly onto and into the stirred, subfluidized bed of polymer forming from the polymerization of monomer from the vapor phase in and over such polymer bed. Provision can be made in the multiple section reactor to introduce the catalyst components and quench liquid at different rates into the different sections of the reactor to aid in individual control of the polymerization temperatures and polymer production rates of the various sections. The reactor introduces monomer or a mixture thereof and, optionally, hydrogen largely or wholly underneath the polymer bed. The polymer solid is continuously removed by passing through a take-off barrier generally at one end of the reactor into a take-off vessel.
Preferably, reactor off-gases are removed along the top of the reactor after extracting entrained polymer fines as completely as possible from the off-gases. The reactor off-gases are then taken to a separation zone where the quench liquid is at least in part separated along with any further polymer fines and some of the catalyst components from the monomer and hydrogen, if used, which monomer and hydrogen are then recycled to inlets spaced along the various polymerization sections of the reactor and located largely or wholly underneath the surface of the polymer bed. A portion of the quench liquid including the further polymer fines is taken off the separation zone and in major part returned to inlets spaced along the top of the reactor. A minor part of such quench liquid, purified of polymer fines and catalyst components, is fed into a catalyst make-up zone for catalyst diluent so that fresh quench liquid need not be introduced for that purpose.
The reactor disclosed in U.S. Pat. Nos. 3,965,083; 3,971,768; 4,101,289; and 4,129,701 largely or completely solves the above referred to problems related to solution or slurry polymerization and reaps important economic benefits through savings in energy consumption, raw materials, and capital costs. However, the known systems for removal of polymeric product from the reactor have resulted in significant reactor downtime and high maintenance costs.
As generally indicated above, the polymer is continuously removed as particulate by passing through the take-off barrier at one end of the reactor into a take-off vessel. In accordance with one known system, the polymer bed level is maintained at the level of an opening in a retaining weir at the take-off end of the reactor. The polymer overflow spills out into a take-off zone and is intermittently removed as particulate through a double ball-valved lock chamber attached to the take-off end of the reactor. The known system comprises a manual isolation ball valve, an automatic inlet ball valve, a blowcase, and a discharge ball valve. In order to discharge product, the inlet ball valve is opened for a specific time, thereby allowing powder to fill the blowcase. After closing the inlet ball valve, the discharge ball valve then opens, thereby discharging the powder from the blowcase via pressure build-up to a powder separation drum.
The following problems have been experienced with the known system. The inlet ball valve occasionally freezes up while cycling, thereby causing pluggage of the downstream discharge elements such as the discharge ball valve. Repairs usually require a reactor shutdown since the isolation ball valve often freezes open. When the discharge ball valve becomes plugged with polymer, powder then builds up in the blowcase. Without proper cooling, the heat of reaction melts the powder, thereby resulting in a fused lump of polymer causing blowcase pluggage. Since the reactor cannot be reliably isolated, the reactor must be shut down whenever the discharge ball valve or blowcase becomes plugged with polymer. Furthermore, in the known system, particulate flows through the opening in the retaining weir into the take-off zone and falls into the blowcase. Since a powder seal cannot be maintained above the discharge ball valve, a large amount of monomer gas leaves with the polymeric product, thereby resulting in excessive gas blowby. This causes monomer losses, as well as high pressures in the powder separation drum. In one aspect, this invention provides a solution to these operational problems.
Additionally, the reactor is particularly adapted for use with catalysts which have a high enough polymerization yield that catalyst residues need not be removed from the polymeric product during the polymer finishing process. However, the quality of product produced in a horizontal vapor phase polymerization stirred-bed reactor does not always equal that of the slurry process because of the catalyst residue left in the gas phase product. The amount of residual catalyst can be reduced by two techniques, developing an improved high yield catalyst, which is beyond the scope of this invention, and/or increasing the reactor residence time, either by extending the reaction time and thereby decreasing the polymer throughput or, alternatively, by increasing the effective reactor volume. Previously, the effective reactor volume and thus residence time as disclosed in U.S. Pat. Nos. 3,965,083; 3,971,768; 4,101,289; and 4,129,701 has been limited by the weir height. Variation of the polymer bed level to increase residence time has required that the weir(s) be replaced which is economically impractical. In another aspect, this invention provides a solution to the problem of controlling residence time without decreasing the polymer throughput or replacing the weir(s).